Technical Insight
Research Review: Germanium takes the Strain out of GaN-on-silicon epi
If you want to grow n-doped GaN layers on silicon with minimal tensile strain, then consider using germanium, rather than silicon as the n-type dopant.
That’s the key finding of a study by researchers from the Otto-von-Guericke-University Magdeburg, Germany, that have compared the tensile strain of GaN layers doped with both of these group IV elements.
The researchers say that until now, good quality thick n-type doping of GaN-on silicon has not been possible due to edge dislocation climb. This results in an increasing tensile strain during epitaxial growth. In fact, studies have shown that doping GaN-on-Si with silicon enhances edge-dislocation climb. The scientists compared samples grown by MOVPE using precursors for AlN and GaN growth processes. These were diluted silane (100ppm) and germane (10%) in H2 for the silicon and germanium doping processes.
Germanium doping has already been tested by Nakamura in the early 1990s. However, with a lower doping efficiency and as a more costly option to silicon doping, germanium doping has not been established. The recent results obtained by the team headed by Alois Krost, on the other hand, show the absence of edge-dislocation climb in the case of germanium-doped GaN-on-silicon.
Krost points out that using germanium as a dopant opens up the possibilities of realizing a wide range of GaN-on-silicon devices, where thick highly n-conducting layers are required. The researchers also discovered that the formation of SiN at the edge dislocation core is the initiator of an enhanced dislocation climb, even for already tensely strained layers. This is in contrast to previous assumptions that the surface roughness of highly silicon-doped GaN-onsilicon leads to edge-dislocation climb.
Doping GaN-on-silicon with an alternative is and old idea that is just coming into its own. The researchers expect not only an impact for GaN-based LEDs grown on silicon substrates, but also in power electronics. For example in vertical Schottky diodes which could now be grown with a highly conducting n-type layer and a thick, undoped layer on top without any impact on strain development.
